Great game! It can definitely be too difficult at times, like the first appearance of the Trojan Horse in level five. For that stage I recommend increasing your hero's attack power and using at least three cannon towers.
For most non-defense levels, I found it easiest to spam enemies with Bombers with a few Interceptors to knock out enemy ships. The final level was easier than all of the sub-80 difficulty levels. Started out with Physics Lab, Starport with Super Carrier and a couple of Fighter Bays with Interceptors. This was sufficient to win the level without building anything else. Very fun game with some issues that other people have detailed in the comments (hot key for pause, Repair Turret priority, mini map obstructing ability to build, etc).
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